Its told that electrons get excited and go to a higher energy level and form hybridised orbitals like in case of carbon,and then the energy is compensated by bind enthalpy. But my question is that from where do they get energy to excite...?????

'x' gm of a compound A3B2C5 contains 'y' gm of A atoms

 

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how to represent the possible fischer projections for 2-bromohexane